The recent approval of immune checkpoint blockade, such as anti-PD-1, has marked a milestone in cancer therapy. Checkpoint blockade ?reinvigorates? an ?exhausted? anti-tumor T cell response, which can result in a durable clinical response. However, only a fraction of patients respond to immune checkpoint blockade, and it only works in a subset of cancers. Improving the efficacy of checkpoint blockade is of paramount importance and is seemingly within reach but will require a better understanding of the molecules that control the complex interactions of immune cells in the tumor micro-environment (TME) required for effective checkpoint blockade therapy. Chemokines are chemotactic cytokines that orchestrate the migratory behavior and cellular interactions of leukocytes, and therefore have great impact upon anti-tumor immune responses. CXCR3 is a chemokine receptor for the interferon-inducible chemokines - CXCL9, CXCL10, and CXCL11- and is highly expressed on CD4+ Th1 cells and CD8+ T effector (Teff) cells. CXCR3 ligands have been correlated with the presence of Teff within tumors and disease free survival. We have exciting data that CXCR3 is required for anti-PD-1 immunotherapy. Based on the importance of CXCR3 for T cell recruitment to sites of inflammation, it is logical to predict that CXCR3 plays an important role in Teff entry into tumors following anti-PD-1 therapy. However, recent provocative preliminary data leads us to believe that CXCR3 is playing even more important roles within the tumor following anti-PD-1, and is likely critical to ?jump start? the anti-tumor immune response in the TME. Recent studies have revealed heterogeneity in exhausted T cell (Tex) populations and defined Tex subsets that differ in their potential for reinvigoration by PD-1 blockade. We have found that CXCR3 expression on Teff inversely correlates with markers of exhaustion. We hypothesize that CXCR3 plays a functional role in the ability of Tex to become reinvigorated within the tumor following PD-1 blockade. In Aim 1, we will define the mechanisms by which CXCR3 contributes to the efficacy of PD-1 blockade therapy for cancer. This will include examining whether CXCR3 plays a critical role enhancing the interaction of Tex with the most relevant activated antigen-presenting cells in the tumor and facilitating the ability of Teff to locate and kill cancer cells following anti-PD-1 therapy. In Aim 2, we will determine if augmenting the CXCR3 chemokine system can improve the efficacy of anti-PD-1 therapy as well as convert anti-PD-1 nonresponsive tumors into responsive tumors. We will also determine if counter-regulatory mechanisms within the tumor, such as epigenetic silencing and CXCR3-expressing regulatory T cells, limit the effectiveness of anti-PD-1 therapy by suppressing CXCL9 and CXCL10 expression in tumors. If these pathways limit CXCR3+CD8+ T cell function in the tumor, we will devise strategies to circumvent these counter-regulatory responses. Finally, we will determine if the CXCR3 chemokine system can be used as a biomarker for response to anti-PD-1 therapy in a murine model and in patients with cancer.